1. The Field of the Invention
The present invention relates to stationary anode assemblies used in certain types of x-ray tubes. In particular, the present invention relates to a stationary target anode that improves the quality and intensity of the x-ray signal generated by the x-ray tube.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials testing.
The basic operation for producing x-rays in the equipment used in these different industries and applications is very similar. X-rays, or x-radiation, are produced when electrons are produced and released, accelerated, and then stopped abruptly. Typically, this entire process takes place in a vacuum formed within an x-ray generating tube. An x-ray tube ordinarily includes three primary elements: a cathode, which is the source of electrons; an anode, which is spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode; and some mechanism for applying a high voltage for driving the electrons from the cathode to the anode.
The three elements are usually positioned within an evacuated tube, and connected within an electrical circuit. The electrical circuit is connected so that the voltage generation element can apply a very high voltage (ranging from about five thousand to in excess of hundreds of thousands of volts) between the anode (positive) and the cathode (negative). The high voltage differential causes a stream, or beam, of electrons to be emitted at a very high velocity from the cathode towards an anode target portion of the anode assembly. The anode target typically is comprised of a metal so that when the electrons strike the target, the kinetic energy of the striking electron beam is converted to electromagnetic waves of very high frequency, i.e., x-rays. The characteristics of the x-rays that are produced, for instance wavelength, depend on the type of metal used for the anode target material. Different metals will produce x-rays having different characteristics. The resulting x-rays emanate from the anode target, and are then collimated onto an object, such as an area of a patient""s body or an industrial device. As is well known, the x-rays that pass through the object, or that fluoresce from the object, can be detected and analyzed so as to be used in any one of a number of applications, such as x-ray medical diagnostic examination or material analysis procedures.
In some x-ray devices, the anode target is positioned on a rotary disk that rotates during operation. Rotation of the anode target reduces the amount of heat present at a particular point on the target at any given time. Other x-ray tubes however, for example certain types used in devices for analytical work such as x-ray fluorescence and x-ray diffraction, use a stationary target anode assembly.
FIG. 1 illustrates one example of a portion of an x-ray tube device 8 that utilizes a stationary anode assembly 10. The stationary anode assembly 10 includes an anode substrate 12 portion, and an anode target 14 that is affixed to the target end 16 of the substrate 12 by a brazing interface 18 or the like. X-ray tube device 8 also includes a cathode assembly, shown as comprising a shield 24 and a filament 25. In operation, an electrical current is passed through the filament 25, which heats up and then discharges a cloud of electrons. As noted, a large voltage potential is placed between the cathode and the anode, which causes the electrons to accelerate to extremely high speeds towards the anode. When the accelerating electrons impinge upon the surface 20 of anode target 14, x-rays are produced, schematically represented at lines 26. Preferably, the x-rays 26 are directed through a window 28 formed on the x-ray tube device 8 and towards an x-ray subject.
The generation of quality x-rays is dependent on several factors, including the type of materials used on the anode target 14, and the physical orientation of the anode target with respect to the cathode. For example, the anode target layer 14 is made from a metallic material having a specific atomic number (Z), which is capable of efficiently generating x-rays when impinged with the high velocity electron stream. In contrast, the underlying anode substrate 12 portion is typically constructed of a different type of metal than the target. For example, copper is often used as a substrate. The selection of this substrate material is based upon several factors. First, its ability to efficiently conduct and dissipate the heat created at the anode target 14 as a result of the impinging electrons is important. Second, the substrate material used is often different from the target material due to the fact that target materials are typically very expensive, and are difficult to machine and manufacture. Thus, use of a different material in the substrate is usually more practical. However, use of a different material for the substrate can give rise to other problems. For instance, the substrate material will emit characteristic x-rays that are different from those emitted from the target. As such, if the anode substrate is impinged with electrons, it is typically a contaminating source of x-rays that can adversely interfere with the x-rays emitted from the target. The x-rays that are emitted from a substrate can be destructive in other ways as well. For instance, in an x-ray fluorescence device, x-rays must be produced from an anode target material that is different from the type of material being analyzed, or the resulting analysis would be inconclusive. Thus, if the substrate material is the same as the material being analyzed, any x-rays generated at the substrate would be destructive.
Generating x-rays that have a specific and consistent wavelength and intensity also requires that the cathode be oriented with respect to the anode target 14 in an appropriate manner. For instance, the filament 25 must be positioned relative to the anode assembly 10 in a manner so that the electrons within the electron stream strike the anode target and thereby generate x-rays. At the same time, the distance (denoted as xe2x80x9csxe2x80x9d in FIG. 1) between the stationary anode assembly 10 and the cathode shield 24, must be large enough to prevent an electrical short from occurring between the anode and the cathode.
Attempts to maintain an optional distance xe2x80x9csxe2x80x9d may, however, give rise to other circumstances that can adversely affect the quality of the x-rays generated. For instance, some electrons from the electron stream 22 may have a primary impact upon face 20 of anode target 14 without producing any x-rays. These electrons can then rebound from face 20 of the anode target layer 14 and result in a secondary electron stream (designated at 30 in FIG. 1) that can impinge upon the anode substrate 12 portion of the anode. As noted, the substrate material that is used to construct the anode substrate 12 is a contaminating source of x-rays. As such, this secondary impact stream may result in the production of an errant x-ray beam (denoted at 32 in FIG. 1), the characteristics of which are often significantly different from the primary x-ray beam 26. As noted, the interaction between the errant beam and the primary beam can adversely affect the quality, the intensity and the focusing of the x-rays that are ultimately produced and released by the x-ray tube device 8, which can ultimately affect the quality of any resulting analyses obtained via the x-rays.
Preventing the formation of such errant x-ray signals has proven difficult. One approach has been to reduce the size of the x-ray device window to prevent the errant x-ray signal from exiting the device and interfering with the primary x-ray signal. However, this approach may also limit the amount of primary x-rays that can exit the x-ray device to an unacceptable level, and thus may not be a viable alternative for certain x-ray applications. Thus, there is a need in the art for a stationary anode assembly that overcomes the problems associated with electrons striking the anode substrate material and producing a low quality, errant x-ray. Moreover, any solution should not affect the overall quality and quantity of the primary x-ray signal that is emitted from the x-ray device.
The present invention has been developed in response to the present state of the art, and in particular, in response to these and other problems and needs that have not been fully or completely solved by currently available stationary anode assemblies for use in connection with x-ray tubes. Thus, it is an overall object of the present invention to provide a stationary anode assembly configuration that improves the quantity and the quality of x-ray signals emitted from the x-ray tube device. Another objective is to provide a stationary anode that reduces the amount of secondary, or errant x-rays from being generated at the anode substrate portion of the anode assembly. Yet another overall object of the present invention is to provide a stationary anode assembly that has an anode target that has a unique geometric shape that functions to reduce the number of electrons that rebound from the target face to the anode substrate. Another object of the invention is to provide a stationary anode assembly having an anode target geometry that also functions to block, or xe2x80x9cshadowxe2x80x9d x-rays that are inadvertently generated at the target substrate from being emitted from the x-ray tube. Another object is to provide a stationary anode assembly having an anode target that causes rebounding electrons to be directed towards the centerline of the target, and away from the underlying anode substrate.
In summary, the foregoing and other objects are achieved with a stationary anode assembly that eliminates, or at least reduces, x-rays from being produced at the anode substrate, thereby improving the quality and intensity of the x-ray signal that is emitted from the x-ray tube device. Embodiments of the present invention utilize an anode target structure that not only provides a suitable target surface, but that also includes means for preventing rebounding electrons from striking the underlying anode substrate. Preferably, the target structure also functions to block or shadow the underlying substrate in a manner such that in the event that x-rays are produced at the substrate, at least some of them are precluded from escaping the x-ray tube. Moreover, other embodiments include means for directing rebounding electrons in a direction towards the center of the anode target surface, and thus away from the underlying substrate. Both features thereby prevent, or at least reduce, x-rays from being produced at the anode substrate, and thus help ensure that a higher quality x-ray signal is emitted from the x-ray tube device.
The present invention contemplates various target configurations and geometric shapes for providing the above functions. For example, in currently preferred embodiments the means for preventing rebounding electrons from striking the substrate is implemented by way of a target anode xe2x80x9coverhangxe2x80x9d structure. The overhanging structure extends out over the underlying substrate, and therefore functions so as to block many of the rebounding electrons from striking the substrate, and also functions to shadow at least some of the x-rays that are inadvertently produced at the substrate from reaching the window of x-ray device. While not required, the overhang is preferably constructed with smooth surfaces and without any sharp angles in the edges. This improves the x-ray generating characteristics of the target anode, and ensures that x-rays are properly focused and directed through a x-ray device window.
In other embodiments, the means for directing rebounding electrons in a direction towards the center of the anode target surface is implemented by way of an anode target surface that has a specific contour in relation to its exposure to the x-ray tube window. This contoured surface can have various geometric shapes, each of which has the advantage of causing target-collided electrons that have yet to generate an x-ray beam to rebound substantially toward the center line of the anode target surface and away from the underlying anode substrate.
In other preferred embodiments of the present invention, a target anode with an overhang portion is combined with an appropriately contoured target surface. This combination provides the advantage of intensifying the occurrence of x-ray beam-generating electrons that strike the target, while still diminishing the occurrence of rebounding electrons that strike the anode substrate and also shadowing any x-rays that are produced from reaching the window of x-ray device.
These and other objects, features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.